The effect of cyclic hormonal changes on the lipolytic enzymes in postheparin plasma was studied in 24 young women. The activity of hepatic lipase was significantly suppressed in the luteal phase of the menstrual cycle in 14 women with "normal" (greater than 3.2 micrograms/L) progesterone levels, which was attributed to the preceding midcycle peak of estrogen secretion. Ten subjects with "deficient" luteal phase levels of progesterone exhibited no significant fluctuations in hepatic lipase activity. No cyclic variation occurred in postheparin plasma lipoprotein lipase activity. Significant but inconstant variations also occurred in some lipoprotein fractions, including a reduction in LDL cholesterol in the luteal phase. However, the changes in lipoproteins were not related to lipolytic enzyme activities, nor could they be attributed to hormonal changes with any certainty.